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March 15, 2006

Dark Matter Lighting up the First Stars

by Sam Savage

Max Planck -- Dark matter could light up the first stars in the universe if the dark matter is made up of sterile neutrinos.

According to a paper by Peter Biermann, (Max-Planck-Institut fr Radioastronomie, Bonn) and Alexander Kusenko (University of California at Los Angeles), recently published in "Physical Review Letters", sterile neutrino decays speed up the formation of molecular hydrogen and light up the first stars as early as 20-100 million years after the Big Bang. The light from these first stars then ionizes the interstellar gas by 150-400 million years after the big bang, in accordance with the observations.* *

The discovery of neutrino masses in neutrino oscillation experiments suggests the existence of right-handed or "sterile" neutrinos, which do not participate in weak interactions directly, but which do interact through their mixing with ordinary neutrinos.

The number of sterile neutrinos is not known, but if one of them has mass of a few keV (one millionth of the hydrogen atom's mass), it can account for the mysterious mass missing in the universe, the "dark matter". Astrophysical observations support the view that dark matter is likely to consist of such sterile neutrinos.

The new theory can explain different astronomical puzzles which remained unexplained so far.

First, sterile neutrinos can be produced in the Big Bang in just the right amount to account for dark matter.

Second, these particles would explain the long-standing puzzle of pulsar velocities: the pulsars (or rapidly rotating neutron stars) are emitted from a supernova explosion preferentially in one direction, hence giving the neutron star a push, much like a rocket engine. Pulsars are known to have velocities in the hundreds of kilometers per second, some in excess of 1000 km/s.

The origin of these velocities remains unknown, but the emission of sterile neutrinos would explain the pulsar kicks. A very fast pulsar in the "Guitar Nebula" is shown in Fig. 1. If dark matter is made of particles which reionized the universe according to Biermann and Kusenko, the same particles streaming from a supernova would have created this cosmic guitar.

Third, sterile neutrinos can help explain the absence of antimatter in the universe. In the early universe, sterile neutrinos could have "stolen" the so called lepton number from plasma. At a later time, the lack of lepton number was converted to a non-zero baryon number. The resulting asymmetry between baryons (like protons) and antibaryons (like antiprotons) is the reason why the universe has no antimatter.

"The formation of central galactic black holes, as well as structure on subgalactic scales favor sterile neutrinos to acount for dark matter. The consensus of several indirect pieces of evidence leads one to believe that the long sought-after dark-matter particle may, indeed, be a sterile neutrino", says Peter Biermann.